1. Field of the Invention
The invention relates to the field of earth boring or coring methodologies and in particular to coring methodologies in the petroleum arts where a sponge jacket is disposed about the cut core.
2. Description of the Prior Art
The loss or migration of fluids, oil or gases from cores cut from deep rock formation is well known and a number of technologies have been developed in order to prevent the loss or migration of such fluids. An alternation of the distribution of fluids in the cut core from that which exists within the core when cut, necessarily involves a material loss of information pertaining to the nature of the fluid or mineral deposition in the rock formation.
One of the prior art technologies which have been developed to preserve the deposition of fluids in cut cores is known as pressure coring. In pressure coring technology, the coring tool includes a means for maintaining the cut core under the same or substantially the same pressure which was exerted on the core when downhole. When the pressurized core is thus retrieved to the well surface, the pressure on the core is maintained and the core is taken to the laboratory where it can be analyzed and depressurized under controlled circumstances.
Another prior art technology for dealing with fluid loss or migration in cut cores is known as sponge coring. In a typical sponge coring tool, an absorbent sponge or foam material is disposed about the cut core such that fluids, which are forced out of the core as the core is depressurized while being tripped, are absorbed in adjacent layers of the cylindrical sponge sleeve. Loss or migration of the fluid or oil in the sponge sleeve is reduced or substantially avoided, thereby permitting additional analysis of the distribution of fluid within the core downhole.
However, sponge coring is susceptible to a number of substantial operational problems. Firstly, the sponge sleeve must be in close or tight contact with the core. This requirement can often be difficult to achieve in broken or unconsolidated cores. Even in the case where the core is hard and consolidated, the necessity of a tight fit between the sponge sleeve and the core can often result in jamming within the coring tool during the coring operation. Secondly, during drilling operations, oil may occur or pool in upper regions or on top of the core. As the core is then disposed within the sponge sleeve, the pooled oil is then deposited in the sponge along the entire length of the core as the core enters the barrel. This is referred to here as oil wipe. Clearly, in such cases, the oil deposition in the sponge sleeve is totally erroneous and masks all information which might otherwise be obtained from the core.
What is needed then is an apparatus and methodology which will permit the practice of sponge coring without susceptibility to jamming or oil wipe characteristic of the prior art.